The invention relates to a high temperature, composite ceramic filter having particular use as a diesel engine exhaust filter, and to a method of forming the same.
As regulatory agencies have recently mandated the reduction of particulate emissions in diesel engines, there has been increased activity in the development of exhaust gas filters for diesel engines. A typical exhaust filter will trap the particulate material contained in the exhaust stream, and to prevent clogging of the filter and the resultant increase of load on the engine due to increased back pressure, the particulate material is then burned from the filter.
As a diesel engine exhaust filter must have high temperature resistance and durability, it has been proposed to utilize ceramic materials as the filter media. For example, a ceramic filter for use in gas turbine engines is described in U.S. Pat. No. 4,017,347. In this patent, a ceramic tape or sheet is prepared from a slurry containing ceramic particles and a two-resin binder system composed of a thermoplastic resin and a thermosetting resin. The tape is formed into a filter structure and the structure is then fired to sinter the ceramic particles and bum out the organic constituents, thus producing a sintered ceramic cellular structure.
U.S. Pat. No. 5,322,537 discloses an exhaust gas filter for diesel engines composed of ceramic fibers, such as alumino-silicate fibers, and an inorganic binder having a softening temperature lower than that of the ceramic fiber.
U.S. Pat. No. 4,652,286 also describes a ceramic exhaust filter for diesel engines having a corrugated or honeycomb structure made of sheets consisting of ceramic fibers and a fire clay binder.
The invention is directed to a high temperature composite ceramic filter having particular use as a diesel engine exhaust filter, and to a method of making the same.
In carrying out the invention, an aqueous slurry is initially produced containing random length ceramic fibers, organic fibers and a water soluble thermoplastic binder. The ceramic fibers, such as alumina, have high temperature resistance, being stable to temperatures above 1000xc2x0 C. The organic fibers can take the form of natural or synthetic materials.
The slurry is formed into a paper-like sheet by conventional papermaking techniques, and the sheet is subsequently dried to evaporate the water and provide a dry flexible sheet.
The sheet is then formed into a green three-dimensional article suitable for filtering. Preferably, the final shape is that of a spirally wound, honeycomb element, composed of flat and corrugated layers, with a colloidal solution of a ceramic material used as an adhesive to join the sheets together along contiguous areas. Opposite ends of alternate channels within the honeycomb structure are sealed by a high temperature cement.
As a feature of the invention, the green filter structure is coated with an intermediate binder to increase its temperature stability. In one form of the invention, the intermediate binder consists of a solvent solution of an uncured thermosetting resin, such as a phenol-formaldehyde resin. The coated part is then air dried and heated to a temperature generally in the range of about 150xc2x0 C. to 250xc2x0 C. to crosslink the resin and rigidify the structure. The structure is then pyrolyzed at a temperature generally in the range of 900xc2x0 C. to 1000xc2x0 C. in an inert or non-oxidizing atmosphere to convert the organic constituents, i.e. the organic fibers and thermoplastic binder, to carbon char. Firing the structure in the inert atmosphere eliminates gassing of the organic constituents and yields a part suitable for final binder application.
A final coating of silicon carbide is then applied to the filter structure using a conventional chemical vapor deposition process. The silicon carbide coats the haphazardly arranged ceramic fibers, as well as the junctions or intersections between the fibers. The resulting structure is a composite of ceramic fibers, inorganic binders and carbon char, coated with silicon carbide.
In a modified form of the invention, the green filter structure is coated with an aqueous colloidal solution of an inorganic material, such as alumina or alumino-silicate binders. The part is then air dried, heated to a temperature of about 200xc2x0 C. to 300xc2x0 C. to remove solvents and dehydrate the colloidal material and then fired at a temperature of 900xc2x0 C. to 1100xc2x0 C. in air to remove the organic components. Following this, the final coating of silicon carbide is applied using the chemical vapor deposition process.
The silicon carbide coating thickness is controlled to a level of about 0.5 to 1.5 microns, so that the porosity of the filter structure is not adversely affected and is maintained at a value of 80% void or greater. Because of the silicon carbide coating, the resulting composite filter has improved mechanical strength, 6,000 kPa or greater having been shown, which is 50-100% higher than can be achieved by bonding or sintering the fibers alone. Additionally, there is no significant degradation or loss of pores within the structure so that resistance to gas flow is minimal.
With the method of the invention, a green state part is produced with a production capable process and the geometry of this part is maintained throughout conversion to a high temperature ceramic composite, and this geometry will be retained at the elevated temperatures of about 650xc2x0 C. to 700xc2x0 C. needed to regenerate a contaminated filter.
Various other features, objects and advantages of the invention will be made apparent from the following description taken together with the drawings.